The present invention relates to an apparatus for controlling the steering angle of a rear wheel of a four-wheel steering vehicle in which the rear wheel is steered in accordance with a steering wheel steering angle, a vehicle speed, and a vehicle yaw rate in a vehicle such as an automobile or the like.
Various four-wheel steering apparatuses for improving steering stability during the operation of a vehicle have been proposed. As disclosed in, for example, Japanese Laid-Open Patent Publication No. 3-164374, an actual yaw rate (rotary angular velocity around the gravity center of a vehicle body as seen from above the vehicle body) is detected by a yaw rate sensor in a steering wheel steering angle by a steering wheel steering angle sensor, and a steering speed of a front wheel is added in the form of a multiplication of the steering speed of the front wheel by a steering control amount in a yaw rate restraining direction. A steering control amount in a yaw rate restraining direction is changed in accordance with a steering speed so that the steering control amount in a yaw rate restraining direction of a steering wheel during a stationary circle turning operation can be made small and an initial rotation during an abrupt steering time of a front wheel may be improved.
In Japanese Laid-Open Patent Publication No. 60-124572, there is proposed a yaw rate following controlling system for calculating a target yaw rate from the steering wheel angle and the vehicle speed, and for steering a rear wheel so that the actual yaw rate may follow the target yaw rate. If a route and a direction of a vehicle go wrong because of external disturbance causes such as a crosswind, a bad road state and so on, due to the yaw rate feedback, any one of the systems has an advantage in that it is capable of steering correction by the rear wheel steering operation.
The conventional four-wheel steering apparatus constructed as described hereinabove takes more time to decide control gains for the calculation of the rear wheel target steering angle so that the side slip angle may be made smaller for safety because of steering wheel angles, yaw rates, and vehicle speeds, which causes a problem in that a control system which is robust, and superior in response with respect to the vehicle speed is hard to design so as to have a simple construction.
The yaw rate sensor is adapted to remove noise through a filter for use in the inputs during the control amount calculation when signals are likely to be influenced by noise so that better precision cannot be directly obtained in the first order differential value, the second order differential value and so on. In order to remove noise of differential values of the yaw rate enough so as to not influence the rear wheel, a cut off frequency of the filter has to be lowered, which causes a defect in that the phase delay of the entire yaw rate feedback loop becomes larger, thus deteriorating control performance. Although a differential value is obtained by a difference equation when a sensor which directly obtains a second order differential value does not exist, the reliability of higher order differential values is lowered considerably.
A stationary target yaw rate also becomes equal to zero when the vehicle speed becomes zero in a target yaw rate following system, which causes a defect in that a smaller turning property during a starting operation cannot be obtained.
A road surface friction coefficient .mu. changes by approximately five times between a dry road surface and a wet road surface, thus changing the transfer characteristics of a vehicle body. When a target yaw rate for the dry road surface is used on a wet road surface, the target yaw rate becomes excessive, thus causing a spinning possibility. When a target yaw rate set on the wet road surface is used on a dry road surface, the yaw rate becomes too small to obtain a satisfactory response.
The same thing can be said even about a control gain C. When a control gain C for a dry road surface is used on a wet road surface, the following of the target yaw rate becomes deteriorated. On the other hand, when an optimum control gain C used on a dry road surface is used on a wet road surface, there is a defect in that safety is deteriorated.
When a steering wheel steering angle is large, the restraining region of a tire goes outside especially on a low .mu. road into a non-linear region so as to lower the steering safety. It is necessary to lower a target yaw rate before the capability of the tire is exceeded.